1. Technical Field
The present disclosure relates to an aluminum alloy conductor used as a conductor of an electric wiring structure, and particularly relates to an aluminum alloy conductor that provides high conductivity, high bending fatigue resistance, appropriate proof stress, and also high elongation, even as an extra fine wire.
2. Background
In the related art, a so-called wire harness has been used as an electric wiring structure for transportation vehicles such as automobiles, trains, and aircrafts, or an electric wiring structure for industrial robots. The wire harness is a member including electric wires each having a conductor made of copper or copper alloy and fitted with terminals (connectors) made of copper or copper alloy (e.g., brass). With recent rapid advancements in performances and functions of automobiles, various electrical devices and control devices installed in vehicles tend to increase in number and electric wiring structures used for devices also tends to increase in number. On the other hand, for environmental friendliness, lightweighting is strongly desired for improving fuel efficiency of transportation vehicles such as automobiles.
As one of the measures for achieving recent lightweighting of transportation vehicles, there have been, for example, continuous efforts in the studies of changing a conductor of an electric wiring structure to aluminum or aluminum alloys, which is more lightweight than conventionally used copper or copper alloys. Since aluminum has a specific gravity of about one-third of a specific gravity of copper and has a conductivity of about two-thirds of a conductivity of copper (in a case where pure copper is a standard for 100% IACS, pure aluminum has approximately 66% IACS), a pure aluminum conductor wire rod needs to have a cross sectional area of approximately 1.5 times greater than that of a pure copper conductor wire rod to allow the same electric current as the electric current flowing through the pure copper conductor wire rod to flow through the pure aluminum conductor wire rod. Even an aluminum conductor wire rod having an increased cross section as described above is used, using an aluminum conductor wire rod is advantageous from the viewpoint of lightweighting, since an aluminum conductor wire rod has a mass of about half the mass of a pure copper conductor wire rod. Note that, “% IACS” represents a conductivity when a resistivity 1.7241×10−8 Ωm of International Annealed Copper Standard is taken as 100% IACS.
However, it is known that pure aluminum, typically an aluminum alloy conductor for transmission lines (JIS (Japanese Industrial Standard) A1060 and A1070), is generally poor in its durability to tension, resistance to impact, and bending characteristics. Therefore, for example, it cannot withstand a load abruptly applied by an operator or an industrial device while being installed to a car body, a tension at a crimp portion of a connecting portion between an electric wire and a terminal, and a cyclic stress loaded at a bending portion such as a door portion. On the other hand, an alloyed material containing various additive elements added thereto is capable of achieving an increased tensile strength, but a conductivity may decrease due to a solution phenomenon of the additive elements into aluminum, and because of excessive intermetallic compounds formed in aluminum, a wire break due to the intermetallic compounds may occur during wire drawing. Therefore, it is essential to limit or select additive elements to provide sufficient elongation characteristics to prevent a wire break, and it is further necessary to improve impact resistance and bending characteristics while ensuring a conductivity and a tensile strength equivalent to those in the related art.
Japanese Laid-Open Patent Publication No. 2012-229485 discloses a typical aluminum conductor used for an electric wiring structure of the transportation vehicle. Disclosed therein is an extra fine wire that can provide an aluminum alloy conductor and an aluminum alloy stranded wire having a high strength and a high conductivity, as well as an improved elongation. Also, Japanese Laid-Open Patent Publication No. 2012-229485 discloses that sufficient elongation results in improved bending characteristics. However, for example, it is neither disclosed nor suggested to use an aluminum alloy wire as a wire harness attached to a door portion, and there is no disclosure or suggestion about bending fatigue resistance under an operating environment in which high cycle fatigue fracture is likely to occur due to repeated bending stresses exerted by opening and closing of the door.
Recently, it is recognized that the following three problems arise when manufacturing an aluminum alloy conductor used for automobiles, particularly an aluminum alloy conductor of around φ0.1 mm to φ1.5 mm. The first problem is that, as has been described above, a high bending fatigue resistance is required when used at a repeatedly bent portion such as a door portion of an automobile. Aluminum has a poor bending fatigue characteristics as compared to currently used copper, and thus locations where it can be used is limited. The second problem is that since it has a high proof stress, installation of a wire harness requires a large force, and a work efficiency is low. The third problem is that since it has a low elongation, it cannot withstand an impact during the installation of a wire harness or after installation, and thus wire breaks and cracks could occur. In order to solve all of these problems, an aluminum alloy wire is required that has a high conductivity as a prerequisite, as well as a high bending fatigue resistance, an appropriate proof stress and a high elongation.
As high strength-high conductivity aluminum alloys, those alloys with Mg, Si, Cu, and Mn added therein are known. For example, Japanese Patent No. 5155464 discloses that adding such elements gives a tensile strength of greater than or equal to 150 MPa and a conductivity of greater than or equal to 40%. Also, Japanese Patent No. 5155464 discloses that an elongation of greater than or equal to 5% is achieved simultaneously by manufacturing a wire rod having a maximum grain size of less than or equal to 50 μm.
However, the aluminum alloy conductor disclosed in Japanese Patent No. 5155464 cannot provide a high bending fatigue resistance and an appropriate proof stress in addition to a high conductivity and high elongation, and thus the three problems described above cannot be solved simultaneously.
The present disclosure is related to providing an aluminum alloy conductor, an aluminum alloy stranded wire, a coated wire, and a wire harness and to provide a method of manufacturing aluminum alloy conductor that provide both an appropriate proof stress and a high bending fatigue resistance while maintaining an elongation and a conductivity equivalent or higher than those of the related art.
The present inventors have found that when an aluminum alloy conductor is bent, a stress occurring at an outer peripheral portion of the conductor is greater than a stress occurring at a central portion, and cracks are likely to occur in an outer peripheral surface. Thus, the present inventors have focused on the fact that, for an aluminum alloy having a smaller grain size, a crack collides with grain boundaries for a greater number of times and thus advances at a reduced advancement rate. The present inventors carried out assiduous studies and found that when an average grain size at an outer peripheral portion of an aluminum alloy conductor takes a value within a predetermined range, an improved bending fatigue resistance is obtained and an appropriate proof stress and a high elongation are further achieved, while ensuring a high conductivity, and contrived the present disclosure.